The present invention relates to a process for the oxidation of hydrogen sulphide dissolved in waste water, containing fine carbon-bearing solid particles, from a coal gasification installation.
In a gasification process such as in the hydrogenating gasification of carbonaceous solid material, a part of the solid carbon-bearing residue, which is often referred to as residual coke, is discharged from the reactor with the product gas. Although a considerable part of that solid material may be removed from the product gas, in a dry condition, by means of a cyclone separator, there is always a certain amount of very fine-grain solid material, with a grain size of from 0 to 0.5 mm and predominantly from 0 to 0.2 mm, that remains in the product gas, and that remaining solid material is washed out by a scrubbing operation using water. However, in that operation, inevitably a part of the hydrogen sulphide contained in the product gas goes into solution in the water, with the result that the slurries that are thus formed by the water with the solid particles suspended therein, besides other compounds, in particular NH.sub.3, phenols and hydrocarbon, also contains hydrogen sulphide.
The gas scrubbing operation is normally performed under the elevated pressure at which the entire gasification system is usually operated, which can be up to 120 bars or even more. The result of that is that, when the slurry leaves the system which is being operated under the above-mentioned elevated pressure, the slurry accordingly undergoes expansion whereupon a part of the hydrogen sulphide dissolved in the waste water of the slurry necessarily goes back into the gaseous phase and is thus separated out of the slurry. At any event however, the waste water still contains a residual amount of hydrogen sulphide which must be removed before the waste water can be discharged for example to a biological purification apparatus, a sewerage system or a main channel of a sewerage system. For that purpose, it is sufficient to oxidise the sulphides in the waste water, as the oxidation levels of the sulphide, with the relative amounts involved, cannot be considered as giving rise to problems for example in a biological purification process or when the material is passed into a sewerage system.
A process has been put forward (German Offenlegungsschrift No. 23 34 994), for the oxidation of sulphide-bearing waste water, also concerned inter alia with the conversion of H.sub.2 S into sulphates, the process being operated at an increased pressure of from 5 to 56 bars and involving the use of a transition metal catalyst. It is accordingly necessary to produce an increased pressure with a consequent not inconsiderable capital expenditure on apparatus, while the use of the transition metal catalyst also gives rise to additional operating costs.
In a further process (German Offenlegungsschrift No. 2654 937) for the removal of phenols and other organic substances from waste water, including waste water from a gasification installation, a fine carbon-bearing material which is removed from a fluidised bed gasification installation by the product gas is used as an adsorption agent, while a stripping operation, in other words a physical process, is carried out to remove hydrogen sulphide.
In another process (German Offenlegungsschrift No. 27 40 706) for treating waste water coal is also used as an adsorption agent and in addition, in connection with the presence of sulphur compounds, an oxygen-containing gas is introduced into the waste water. However, that is effected in order to prevent the production of aerobic microorganisms which in turn can form hydrogen sulphide. Accordingly, that specification presupposes that any sulphur compounds to be removed are not present in the form of hydrogen sulphide, and therefore that process is also not concerned with converting hydrogen sulphide into an environmentally safe form.
Another process (German Offenlegungsschrift No. 23 41 228) for decontaminating cyanide-bearing waste waters comprises using a suspension of ash in water, which is produced by scrubbing out fly ash particles, which do not contain any carbon constituent, from the product gas of a gasifier. In that process, any hydrogen sulphide is removed by chemical reaction thereof with the ash which therefore acts as a reagent.